Recording materials using a combination of an electron donating colorless dye (hereinafter, referred to as "color former") and an electron accepting compound (hereinafter, referred to as "color developer") are well known for use as a pressure-sensitive paper, heat-sensitive paper, light- and pressure-sensitive paper, electric heat-sensitive paper, heat-sensitive transfer paper, and the like. The details for these recording materials are described, e.g., in U.S. Pat Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, and 3,418,250, JP-A-49-28411 and JP-A-50-44009 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, JP-B-60-23922 (the term "JP-B" as used herein means an "examined published Japanese patent application"), and JP-A-57-179836, JP-A-60-123556, and JP-A-60-123557.
These recording materials are required to (1) provide an image having sufficient color density with sufficient color formation sensitivity, (2) cause no fog, (3) provide an image having sufficient fastness, (4) form a hue suited for copying machines, (5) have a high S/N ratio, and (6) provide an image sufficiently resistant to chemicals, and the like. Recent studies have been directed particularly to improvements on the characteristics (1) and (3) above.
However, the conventional recording materials have various problems which may seriously impair their commercial value.
For example, when a pressure-sensitive recording material is preserved in files made of, for example, vinyl chloride, the recorded area undergoes color disappearance due to the action of a plasticizer, etc.
Heat-sensitive recording materials undergo fog upon contact with solvents, etc., and these materials undergo decoloration or discoloration of the recorded area upon contact with fats and oils, chemicals, etc. Namely, when the heat-sensitive recording material makes contact with stationery which contains materials such as aqueous or oily inks, fluorescent inks, stamp inks, adhesives, starch paste, diazo developers, etc., or cosmetics, such as hand creams, emulsions, etc., the white background tends to undergo fogging or the color developed area tends to be discolored.
The inventors researched both color formers and color developers in pursuit of a satisfactory recording material. In observing the use of color formers and color developers, the inventors focussed their attention upon (1) solubility in oil or water, (2) partition coefficient, (3) pKa, (4) polarity of substituents, (5) position of substituents, and (6) change in crystallizability and solubility when color formers and color developers are used in combination, etc.